All organisms belonging to the kingdom Protista are

CONCEPT: Whittaker's five-kingdom classification system (Monera, Protista, Fungi, Plantae, Animalia) groups all living organisms by cell structure (prokaryotic vs eukaryotic) and body organization (unicellular vs multicellular). At this classification level, kingdom Protista is defined as the group of eukaryotic organisms that are treated as single-celled — organisms with a true, membrane-bound nucleus and other membrane-bound organelles, housed in one cell.

APPLICATION: Matching this definition to the item — Protista members such as Amoeba, Paramecium, diatoms, and Euglena each show a true nucleus (eukaryotic cell type) within a single cell (unicellular body plan). Both traits together are what this classification uses to place an organism in kingdom Protista.

CROSS-CHECK: Checking each option pair against the other four kingdoms confirms this by elimination:

• Monera (bacteria, blue-green algae) is unicellular and prokaryotic (no true nucleus).

• Fungi, Plantae, and Animalia are eukaryotic and predominantly multicellular (though a few fungi, like yeasts, are unicellular).

• No kingdom in this system pairs a multicellular body plan with a prokaryotic cell type.

RESULT: Among the four pairings offered, only the unicellular + eukaryotic combination is how kingdom Protista is classified in this system.